Journal of the AES

2002 March - Volume 50 Number 3

Papers

This is the third of a three-part series that provides a tutorial
reference for those signal processing algorithms that are of particular
interest to music. In this part, the issues of low-frequency sinusoidal
oscillators and sonically pleasant pseudorandom noise generators are
reviewed, compared, and analyzed. Both topics actually have deep issues
even though the signal definitions are simple. The historical
background, pertinent references, and appendices provide the reader with
a comprehensive foundation in these subjects.

Because small-volume loudspeakers are unable to reproduce
low-frequencies, alternative methods can be used to create the illusion
of those frequencies by taking advantage of a phantom-pitch
phenomenonthe perception of a fundamental when only its harmonics
are present. The low frequencies below the loudspeaker cutoff point are
extracted from the wide-band signal, nonlinearly processed, filtered
again, and finally injected into the path of the main signals. Listeners
prefer this artificial bass in comparison to flat reproduction through a
small loudspeaker. A particular implementation of phantom pitch is
illustrated.

Most speech-synthesis systems do not provide a mechanism for creating
nonspeechlike signals, such as sounds like boom, meow, gurgles, and
genetic vocal noises. To avoid the mechanical-sounding attributes of
other approaches, the author adapts a cellular automata control of the
spectral parameters. The array of parameters is updated according to a
set of rules to create transitions between states using the values of
the neighboring cells as input parameters. While this approach works
well to create natural sounds, there is insufficient knowledge about how
to create an imagined sound.

Estimating the Loudspeaker Response when the Vent Output is Delayed(PDF-72K)

Neville Thiele

173

A method is proposed for estimating the loss in amplitude response for a
loudspeaker when the acoustic output from the vent is delayed with respect to
the main output. Although the model is simple, it provides insight into the
way low-frequency response is influenced by lumped delay to the vent output.
Designers are often forced to move the vent to a more distant location, and
delay is thereby added to that acoustic path.